Demographic legacies of fire history in an African savanna

Abstract

Summary 1.Fire is a key determinant of woody vegetation structure in savanna ecosystems, acting both independently and synergistically through interactions with herbivores. Fire influences biodiversity and ecological functioning, but quantifying its effects on woody structure is challenging at both species and community scales. 2. Deeper insight into fire effects, and fire-herbivore interactions, can be gained through the examination of species-specific demographic and dynamic changes occurring across areas with different fire regimes in the presence of large herbivores. We used the Carnegie Airborne Observatory (an integrated LiDAR and imaging spectroscopy system) to map woody tree structure, species and dynamics over a four-year interval across two adjacent savanna landscapes with contrasting fire histories in Kruger National Park, South Africa. 3. A history of higher fire frequency was associated with reduced woody canopy cover (17% vs. 23%) and an increased overall rate of treefall (27% vs. 18%). The landscape with a history of higher fire frequency displayed a shift in woody canopy height distribution from a unimodal curve to a bimodal pattern at the community scale, with large reductions in height classes <7 m. 4. Differences in tree height distributions and treefall rates across sites were underpinned by species-specific responses to fire frequency. Acacia nigrescens displayed the highest rates of treefall, most likely related to elephant activity, with losses exceeding 40% in the 6-to 9-m height classes. 5. Synthesis. Our findings indicate that fire history imparts demographic legacies not only on vegetation structure, but also on current vegetation dynamics. Current treefall rates of certain tree species are exacerbated by a history of higher fire frequency. Species-specific and contextconscious investigations are critical for elucidating the driving mechanisms underlying broader community patterns.

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